Particle sorting technologies are widely used for targeting moieties in suspension, but undue contamination can be a technical hurdle to overcome. For example, one may seek to sort relatively rare cells from a complex whole blood sample. Contamination by other cell types may require the need for pre-treatment to enrich for target cells, or post-treatment to remove unwanted non-target cells.
Moreover, contaminating moieties may localize to container sidewalls or other surfaces. Although one may minimize non-specific binding with surface coatings, such as silicone-based products, this may be insufficient. Where there is a horizontal fluid flow plane, and the particles flow in suspension along that plane, they may sink to the bottom (depending on density, viscosity, and other characteristics). Particles may then form a barrier, clogging up the flow path. Plus, contaminating particles may be co-localized in (for example) microfluidic devices configured with a trapping structure. Particularly where target particles are extremely rare, non-specific binding can confound sorting or detection of the target species.
There are ways to enhance particle sorting specificity. In a microfluidic volume, hydrodynamic focusing may be used to transport target moieties. Conventional approaches to hydrodynamic focusing involve using two outer fluidic flows on each side of a central sample flow to laterally constrain the sample flow. See generally, P. Crosland-Taylor, “A device for counting small particles suspended in fluid through a tube,” Nature 171:37-38 (1953) doi: 10.1038/171037b0 for a seminal paper on the subject of flow cytometry, and the use of fluidic sheaths.
In general, sheath flow is a particular type of laminar flow. Although sheath flow may be configured as an outer flow “tube” surrounding a fluid stream, or other fluidic path for total or partial surrounding of a fluid stream, sheath flow herein also includes a fluidic flow path in laminar flow with respect to an adjacent, parallel fluid flow path. Thus, what is a laminar flow plane on a solid surface is considered a sheath flow plane when on a fluid “surface” (e.g., the adjacent, parallel fluid flow path). Sheath flow implies substantially no turbulent flow, as undue turbulence would result in intermixing of fluidic flow paths (and the fluidic laminar flow plane layer would no longer function as a “sheath”). As such, laminar flow, rather than turbulent flow, is necessary to create sheath flow.
Depending on the architecture and fluid characteristics, sheath flow may function to hydrodynamically focus a fluid sample (by surrounding a sample flow path without intermixing), or, where there is a layer (or sheath incompletely surrounding a fluid flow), the fluid in a laminar flow plane may act as a fluidic extension of a device wall—essentially acting as a fluidic barrier between a fluidic sample and surrounding solid surfaces.
Sheath flow is particularly useful in a microfluidic context, where particles in suspension may disrupt or block microfluidic circuitry. Thus, hydrodynamic focusing or sheath flow allows for faster sample flow velocity, and higher throughput.
Nevertheless, creating laminar sheath flow useful for microfluidic devices (or larger devices) is problematic. Conventional devices, such as conventional flow cytometers require complex instrumentation with specialized components, and fabrication is particularly detailed. Although there may be particular geometries and architectures reportedly creating microfluidic sheath flow, there exists a need for easy to manufacture, predictable sheath flow devices useful at the macro- and micro-fluidic scales, particularly for improved sensitivity in target moiety sorting, and reducing non-specific binding to surfaces.